Structural Insight into Guest Binding Sites in a Porous Homochiral Metal–Organic Material

An enantiomeric pair
of chiral metal–organic materials (CMOMs)
based upon mandelate (man) and 4,4′-bipyridine (bpy) ligands,
[Co2(S-man)2(bpy)3]­(NO3)2·guest (1S·guest) and [Co2(R-man)2(bpy)3]­(NO3)2·guest (1R·guest), have been prepared. The cationic frameworks exhibit one-dimensional
chiral channels with dimensions of 8.0 Å × 8.0 Å. The
pore chemistry is such that chiral surfaces lined with nitrate anions
and phenyl groups create multiple binding sites for guest and/or solvent
molecules. The performance of 1S and 1R with
respect to resolution of racemic mixtures of 1-phenyl-1-propanol (PP)
was studied by varying time, temperature, and the use of additives.
Selectivity toward PP was determined by chiral HPLC with ee values of up to 60%. The binding sites and host–guest interactions
were investigated through single-crystal X-ray structural analyses
of guest-exchanged 1S and 1R. Crystallographically
observed structural changes (e.g., the absolute configurations of
the three PP binding sites switch from R, R, and S to R, R, and R/S) correlate
with experimentally observed ee values of 33% and
60% for variants of 1S that contain PP and different
solvent molecules, 1S·PPex and 1S·PPex′, respectively. The fact that manipulation of guest solvent molecules,
which in effect serve as cofactors, can modify chiral sites and increase
enantioselectivity is likely to aid in the design of more effective
CMOMs and processes for chiral separations.

本研究制备了一对基于扁桃酸根（mandelate，man）与4,4′-联吡啶（4,4′-bipyridine，bpy）配体的手性金属有机材料（chiral metal–organic materials，CMOMs）对映异构体对，即[Co₂(S-man)₂(bpy)₃](NO₃)₂·客体（1S·客体）与[Co₂(R-man)₂(bpy)₃](NO₃)₂·客体（1R·客体）。该类阳离子骨架拥有尺寸为8.0 Å × 8.0 Å的一维手性通道。其孔道化学特性为：由硝酸根阴离子与苯基修饰的手性表面可为主客体分子提供多个结合位点。本研究通过调控反应时间、温度及添加剂使用情况，考察了1S与1R对外消旋1-苯基-1-丙醇（1-phenyl-1-propanol，PP）的拆分性能。采用手性高效液相色谱（chiral HPLC）定量分析该体系对PP的拆分选择性，所得对映体过量值（enantiomeric excess，ee）最高可达60%。通过对客体交换后的1S与1R开展单晶X射线结构分析，本研究对其结合位点与主客体相互作用进行了深入解析。晶体学观测到的结构变化（例如：3个PP结合位点的绝对构型从R、R、S转变为R、R与R/S），与分别采用不同溶剂分子（1S·PPex与1S·PPex′）负载PP的1S变体实验测得的33%与60%对映体过量值高度吻合。研究表明，通过调控本质上可作为辅因子的客体溶剂分子，能够修饰手性结合位点并提升对映选择性，这一发现将有助于设计更为高效的手性金属有机材料与手性分离工艺。